Liquid chillers, such as water chillers, are well known. A conventional water chiller includes a water reservoir and a method for cooling the water contained in the reservoir. Water chillers are typically found, for example, in athletic clubs, offices, and the home, and provide a cool liquid refreshment. In addition to providing liquid refreshment, liquid chillers may also be used to cool an ambient environment and have found applications in, for example, the semiconductor industry.
Liquid chillers may be divided into two categories. A first category of liquid chillers is characterized by a large thermal capacity that can quickly chill all of the liquid contained in an associated liquid reservoir. Such chillers may use, for example, compressors with a simple on/off control to cool the liquid. A second category of chillers is characterized by a relatively small chilling capacity in comparison to the thermal requirements of an associated liquid reservoir. One example of this second category of chillers is a thermoelectric-based cooling system. Another example is a compressor-based cooling system that includes an undersized compressor.
In chillers having a relatively small chilling capacity, it is often desirable to form solid material, such as ice, as a method of storing thermal energy for peak demand periods. For example, a water cooler may experience higher demand for cooling during a particular portion of the day than at other portions of the day. Conventional thermoelectric-based chillers may not be able to cool water received by the water cooler quickly enough to provide water that is cool during the entire peak period. To combat this problem, during off-peak times the cooler operates to form ice within a portion of the water reservoir. This ice formation allows for storing of energy that is later used during peak periods to cool the water in the liquid reservoir. During peak periods, as the liquid is consumed, ambient temperature water replaces the chilled water that has been consumed from the water reservoir. During peak periods, the ambient temperature water is more quickly cooled by the combination of the ice and the thermoelectric cooler than cooling that would take place in the absence of the ice. The formation of ice allows a large amount of energy to be stored because of the large amount of energy associated with a phase change from water to ice. Therefore, in this manner, lower capacity chillers may provide chilled liquid to users both during peak periods and off-peak periods. Such a system, therefore, reduces the size of a compressor that might be required or allows the use of alternative cooling systems, such as thermoelectric-based systems.
Although formation of ice allows the storage of energy for later use during peak periods, the formation of ice also creates problems. For example, operating a thermoelectric cooler or a compressor-based cooler in a manner that creates ice may cause the entire liquid reservoir to freeze, thus, in the example of a water cooler for providing a cool potable liquid, restricting the ability of users to receive liquid refreshment upon demand. In addition, particular portions of the liquid reservoir may freeze, such as portions near a fluid outlet, also inhibiting users from receiving liquid refreshment on demand.
In order to attempt to overcome these problems associated with the formation of ice as a method of storing energy for peak demand periods, several control systems have been developed. In one control system, the temperature of a cooling element is controlled based on the amount of ice formed. Sensors, such as optical sensors, may be used to detect the formation of ice, and the control system may appropriately adjust the temperature of a cooling element when a desirable amount of ice has been formed. However, such a system may experience disadvantages. For example, once ice is formed, reliably controlling the temperature of the cooling element and additional ice formation is difficult. In another type of control system, optical sensors detect the presence of ice on a cooling element, and ice is discharged from the surface of the cooling element after the ice has formed. Such discharge allows better control of ice formation on the cooling element because the insulative effect of the ice on the cooling element is removed as a complicating parameter. However, such systems do suffer the problem of filling the reservoir with ice, which may inhibit discharge of liquid refreshment to a user on demand or otherwise adversely affect the performance of liquid chillers used to cool an ambient environment.